In the art of sound reproduction systems, it is well known that the location of transducers, often referred to as loudspeakers, has a substantial affect upon sound reproduction of stereophonic signals. Accordingly, speakers are preferably arranged in order to produce psychoacoustically pleasurable sounds to the area occupied by the listeners. However, particularly in motor vehicles, the number and position of the speakers is often dictated by other packaging considerations and cannot be arranged for the sole purpose of providing maximum listening pleasure to the vehicle occupants. Accordingly, there have been several developments to process the signals to be emitted from the speakers in order to adjust the audio reproduction image of a stereophonic signals.
Several attempts have been made to generate signals that simulate a relocation of the speakers as if they had been spread further apart or located in a different direction from the listening area. U.S. Pat. No. 4,329,544 to Yamada discloses a sound reproduction system attempting to audibly simulate a wider distance between the speakers. A transfer function equalizes sound pressures from a signal representative of a third speaker location and the conventional output emitted from stereo speakers. The system also includes a delay circuit in one channel to compensate for the difference in distances between the listener and each of the speakers, and also includes a reverberation circuit. U.S. Pat. No. 4,394,536 also discloses an apparatus for acoustic spreading and reverberation effects for reproduced sound and the effects can be adjusted by the user.
U.S. Pat. No. 4,868,878 to Kunugi et al. discloses a sound field correcting system in which the transfer function adjusts a level in delay of the signal to compensate for the distance between the travel of direct and reflective sound waves to a listening point. U.S. Pat. No. 4,980,914 issued from a continuation-in-part application of U.S. Pat. No. 4,868,878 and discloses the additional feature that high pass or low pass filters may be used as desired at appropriate points of the system.
U.S. Pat. No. 4,980,915 to Ishikawa discloses an integrated circuit switch for use with a system including a center input signal as well as left and right input signals.
U.S. Pat. No. 4,495,637 discloses a method and apparatus for enhancing psycho-acoustic imagery by asymetrically crossfeeding left and right signal inputs. The asymmetry is designed to complement the listener's brain processing of perceived acoustic signals due to naturally occurring left or right half brain dominance of the listener. The system employs out of phase crossfeed without filtering or delays.
U.S. Pat. No. 4,388,494 to Schone et al. discloses a stereophonic reproduction system using the dummy head recording process and a headphone reproduction process with filtering and crossfeeding of the channels.
Like U.S. Patents to Yamada and Kunugi et al., U.S. Pat. No. 4,219,696 to Kogure et al. reproduces sound from two loudspeakers located in front of the listener to generate relocalized sound in a manner that simulates sound reproduction sources to the rear of the listener. The apparatus includes transfer functions canceling sound in the direct path and imposing a time difference between sound waves applied to the left and right ears of the listener. Similarly, U.S. Pat. No. 4,192,969 to Iwahara discloses a stereophonic sound reproduction system simulating an expanded stage by crossfeed paths between the channels with a first transfer function representative of ratio of the crossfeed transfer function to the direct transfer function corresponding to a hypothetical sound location with respect to the listener's ears, and a second transfer function corresponding to the ratio of crossfeed transfer function to direct transfer function corresponding to the actual sound direction.